My invention relates to automated metrological machines used for optical inspection of an object. Metrological machines are typically employed for the automated optical inspection of manufactured objects, and are particularly useful in determining the precise dimensional measurements of such objects. They normally include a support on which the work object rests, and means for precisely moving either the object or an imaging video camera that is used for recording and/or displaying a magnified image of the object that is being inspected. These features allow such machines to perform precision measurements in the horizontal (or X-Y) plane. Autofocus means can also be included for determining heights of the object along the “Z” axis normal to the X-Y plane, enabling a full, three-dimensional inspection of the object.
A good example of an advanced metrological machine system is provided by U.S. Pat. No. 6,292,306, for a “Telecentric Zoom Lens System for Video Based Inspection System” (2001). In this patent, the object to be inspected is positioned on a support surface beneath a series of imaging lenses that are secured to a system support that overlies the support surface and object. The lenses have an optical axis disposed vertically and the system includes a coaxial adjustable telecentric stop or iris diaphragm and two moving groups of lenses for performing magnification zooming, where these are controlled by cam and slot means as in conventional zoom lenses. (See, also, U.S. Pat. No. 5,389,774 describing a system allowing a user to perform calibration and return to a previously saved magnification by saving a reticle image at a selected magnification, calling up that image when that magnification is desired again, and waiting for the zoom lens to adjust until the present reticle image matches the saved reticle image).
The complete system may also include a substage collimator or illuminator to offer a silhouette image of the object as further described in U.S. Pat. No. 6,488,398, and an LED ring surface illuminator. The LED ring surface illuminator, as further described in U.S. Pat. Nos. 5,690,417 and 6,179,439, allows contours, ledges, edges and other generalized surface height variations to be imaged. Finally, and most importantly for the purposes of this invention, a “beam splitter” within the lens system can be used to inject surface inspection illumination from an illumination source transverse to the optical axis along the optical axis through the lens (“TTL”). (See, e.g., “illuminator light source S” in U.S. Pat. No. 6,292,306 and its accompanying drawing figures).
The TTL illumination allows microscopic surface details to be imaged. For objects with little or no contrast, a TTL grid illumination source can also be used along with the TTL surface illumination source. The grid is projected onto the surface of the object and is illuminated by a separate light source transverse to the optical axis of the TTL illumination system, reflected off of a separate beam-splitter on the optical axis of the TTL illumination system, and projected through the front lens onto the object to be imaged as discussed in the preceding paragraph.
However, while beam splitters make the aforesaid TTL illumination systems possible, they also lead to certain problems. Beam splitters typically take the form of a half-silvered mirror, or a cube made from two triangular glass prisms. The mirror-type (or plate) beam splitter is an optical window with semi-transparent mirrored coating used to separate a single beam into two beams. Beam splitter cubes are more advanced beam splitters consisting of two right-angle prisms cemented together at their hypotenuse faces. The hypotenuse face of one prism is coated with a metallic or dielectric layer having the desired reflecting properties. Thus, when the plate beam splitter is arranged at a 45 degree angle to the optical axis, a portion, typically half, of the incipient light traveling along the optical axis will be reflected and the rest transmitted. Likewise, when a 50/50 cube beam splitter is arranged along the optical axis such that two faces of the cube are normal to the axis and the hypotenuses of the two triangular glass prisms are at a 45 degree angle to the optical axis, half of the incipient light traveling along the axis will, once again, be reflected and half transmitted.
Thus, as will be appreciated by those skilled in the art, while each beam splitter can be used to insert useful illumination along an optical axis for TTL illumination purposes, it also will divert and thereby diminish light already traveling along that optical axis. Consequently, the inclusion of both a TTL surface illumination system and a TTL grid illumination system in the same metrological machine system (where one is oriented to project light directly along the illumination system's optical axis and the other injects light transverse to that axis) generally leads to light level problems.
Typically, the TTL surface illumination system is oriented to project light along the illumination system's optical axis with an additional beam splitter for the TTL grid illumination system, as previously described, in the optical path for TTL illumination intermediate the target object and the respective light sources. Where, for example, a 50/50 beam splitter is used in this role (as is typical), neither system has truly adequate illumination.
I have, therefore, sought and invented a system that, instead of using a 50/50 beam splitter of the types outlined above to inject TTL grid illumination, uses two right angle triangular prisms (similar to those used in a cube beam splitter) that are aligned to each other along their respective hypotenuses (like a cube beam splitter), but without gluing the two triangular sections together, without any reflective materials being used at their interface, and with the two halves separated by some distance so as to create an open/air space between the two halves. When the upper/lower cube faces are normal to the optical axis of the illumination system, the system reflects virtually all of the illumination from the TTL grid illumination source through the lens (“TTL”) onto the work object due to total internal reflection (“TIR”). However, turning the “cube” a mere 12 degrees, past the critical angle, eliminates total internal reflection and allows almost all of the light from the TTL surface illumination system to pass through the “cube” to illuminate the work object.
Thus, by utilizing the principle of TIR, I allow nearly 100% of the reflected light for imaging the grid to exit the illumination system for illumination of the work object and, by rotating the “cube” slightly past the critical angle, I allow nearly 90% of the light from the TTL surface illumination source to exit the illumination system for work object illumination. The shift from one mode of illumination to the other is controlled, like other aspects of the metrological machine, by computer and can be quickly, simply and easily effected by actuation systems well known in the mechanical and optical arts, thereby solving the previously described problem of inadequate TTL surface and grid illumination for systems of this type.